This invention relates generally to a filter, for thickening suspensions of fibrous material, with rotary filter elements encased in a filter trough in which filter the said filter elements comprise pairs of filter disks fixed to a rotating hollow shaft and the chamber formed inside a pair of such filter disks is connected to the hollow shaft and wherein the hollow shaft is connected to a suction head, to which downpipes are connected, with material outlet shafts being disposed between the pairs of filter disks.
Filters of this kind are normally used in the paper industry as so-called disk filters for recovering fibrous material. To this end a plurality of pairs of filter disks are disposed next to one another on the hollow shaft in a filter trough. Because of the high pressure difference caused by the downpipes between the fibrous suspension in the trough and the inner chamber formed by the pairs of filter disks, a thick fleecy coating of fibrous material exhibiting about 10 to 12% atro forms very rapidly on the individual disks.
With disk filters the process for recovering the fibrous materials occurs in the following manner: After immersion of the filtering faces, the fibers settle on the surface of the filters while the so-called turbid water, water mixed with fibers, still passes through the filtering face (first water extraction zone). But a tight fleece of fibers forms very rapidly, this serving as an auxiliary filtering layer and only allowing clear water to pass through the filter now (second water extraction zone). Here the fiber thickening only takes place on the surfaces of the filters.
This fiber layer is then sprayed from the disk faces into the material outlet shafts with water, preferably turbid filtrate, from the first water extraction zone. In addition a clear water of very high quality is obtained in the second water extraction zone.
So-called drum thickeners are known in the paper industry for simple thickening work. In these thickeners the drum shell acts as a filtering face. But no particularly thick fibrous coating forms on the drum shell. Rather the suspension is thickened in the trough itself. The pressure drop is generally constituted by the difference in level between the fluid inside the drum and the suspension in the trough. With this, concentration values of 4 to 7% atro are reached.
To improve the thickening action with drum thickeners it is already known (as disclosed by German Pat. No. 1,254,594) to employ stirring devices incorporated in the drum so that the suspension is swirled around and the solid particles are carried to the wall of the filter drum. In spite of the small filtering area as compared with disk filters, drum thickeners generally have a thickening performance of about 2 to 2.5 tonnes/day/m.sup.2 of filtering area. But in the case of disk filters the thickening performance is only about 0.5 to 0.7 tonnes/day/m.sup.2 of filtering area. The advantage of a larger filtering area is partially eliminated again in the case of disk filters by the lower output of the disk filter. This is because the rapid fleece formation on the surfaces of the filter only allows minimal further water extraction. For this reason disk filters have largely been used for fiber recovery, up to now, since the throughput was of lesser importance than optimum fiber recovery.
Consequently, the general object underlying the present invention is to raise the throughput levels in disk filters so that disk filters can be used rationally not only for fiber recovery but also for thickening suspensions of fibrous material.